Integrate initial release of Context service

See merge request teraflow-h2020/controller!3

Move database to common folder, since it will be used also by device service, and implement very minor changes.

- monitoring/test_monitoring.py: provides a basic testing skeleton
- monitoring/requirements.in/.txt: updated with pytest
- monitoring/genproto.py: added instructions to import generated Python modules using "from . import ..." instead of "import ..."; otherwise, pytest and other possible modules doing imports might complain.
- monitoring/monitoring_server.py: packed start_server and stop_server functionaity to simplify test definition, minor formattings, bug resolutions and improvements in package import.
- monitoring/monitoring_client.py: minor formattings, bug resolutions, and improvements in package import.

- monitoring/requirements.txt: removed garbage on top of file
- monitoring/monitoring_client.py: solved bug in return values of some methods and wrong code in test case
- monitoring/monitoring_server.py: solved bug with instantiation of servicer class
- monitoring/Dockerfile: clarified some arguments in package installation, moved code in container to /var/monitoring, added upgrade of essential python packages, other minor improvements
- monitoring/context_pb2.py: upgraded version since previous has no contextId fields.